This invention relates to a liquid crystal display device, particularly to a liquid crystal display device of the twisted nematic type (hereinafter sometimes referred to also as "TN type") which is suitable for a multiplex driving system (a time sharing driving system).
Liquid crystal display devices find applications primarily of the TN type in uses which are now expanding for having such advantages that power consumption is small, that production cost is low, that it can be made lightweight and thin, that it is easily applicable for color display, etc.
A liquid crystal display device of the TN type is constituted by sealing a nematic liquid crystal having a positive dielectric constant anisotropy between two sheets of electrode substrates subjected to aligning treatment, and is generally made under the state in which the liquid crystal molecules are sequentially twisted by 90.degree..
The liquid crystal display device of this type with a twist angle of 90.degree. (hereinafter sometimes called as "90.degree. twisted type") can be produced simply and is suitable for mass production, and also has the advantage of rapid response.
However, the liquid crystal display device of the 90.degree. twisted type is moderate in intensity change of transmitted light (or reflected light) to the change in applied voltage, and therefore has the problems such that the contrast is low to give sharp images with difficulty and also the viewing angle is narrow when the time sharing order is made greater in multiplex driving, thus being limited in application for high multiplex driving system. For example, in a liquid crystal display device with the display area of about A4 size, the duty ratio in multiplex driving has been practically preferred to be 1/200 or higher, but said duty ratio practically applied in the liquid crystal display device is about 1/100, with its contrast ratio (ratio of luminance on selected elements to that on unselected elements) being as low as about 3.
As a technique for solving the problems of such a 90.degree. twist type liquid crystal display device, Japanese Provisional Patent Publication No. 107020/1985 discloses a liquid crystal display device characterized in that the twist angle of liquid crystal molecules is 180.degree. to 360.degree. and the angle between the director direction of the liquid crystal molecules alignment on at least one of the electrode substrates and the surface of the electrode substrate (hereinafter sometimes referred to also as "pretilt angle") is greater than 5.degree.. According to this liquid crystal display device, since the intensity change of transmitted light to the change in applied voltage is steep, it is stated that a high contrast ratio of 19.6 can be realized in the case of multiplex driving at a duty ratio of 1/100.
However, in this liquid crystal display device, no sufficient consideration has been given to the bistable effect and, for this reason, there is the problem that response is slow when the liquid crystal display device is subjected to multiplex driving at a high duty ratio. More specifically, in the liquid crystal cell of this liquid crystal display device, there ordinarily occurs the so-called hysteresis phenomenon in which the intensity change of transmitted light or the reflected light during increasing the applied voltage is different from that during decreasing of the applied voltage. Due to the bistable effect which causes this phenomenon, there are involved the problems such that the practical voltage range during time sharing driving may be narrowed or the response time for on-off may be elongated. Also, as the result of these problems, the display badness is liable to occur by non-uniformity of the liquid crystal layer thickness, the temperature change, etc. For this reason, it becomes necessary to suppress the bistable effect as small as possible.
For such reasons, the present inventors have proposed a liquid crystal display device characterized in that the twist angle of the liquid crystal molecules aligned between a pair of substrates having alignment layers respectively is 180.degree. or more and 360.degree. or less, and the relationship formula of 0&lt;(Pc-Ps)/Pc&lt;0.3 is valid between the spontaneous twist pitch Ps of the liquid crystal molecules and the regulated twist pitch Pc of the liquid crystal molecules when the arrangement of the liquid crystal molecules is compulsorily regulated by the alignment layer (see Japanese Patent Publication No. 127714/1987 which corresponds to U.S. Ser. No. 06/935,225 now in pending).
According to such a liquid crystal display device, the bistable effect of a liquid crystal cell can be suppressed to some extent to give a result that the margin in the change of driving voltage can be made greater to effect considerably improvement in contrast ratio and response speed.
However, in these days, it has been desired to have a liquid crystal display device of further quick response to necessitate further improvement of the above technique.